It’s estimated an average of one passenger in 6,000 will suffer from venous thromboembolism after a long-haul flight. If left untreated, about 1 in 10 people will develop a pulmonary embolism, where part of the clot breaks off and becomes lodged in the lungs.

■ What you can do: Choose loose-fitting clothes, stretch and flex your leg muscles regularly, walk around the plane when allowed, wear well-fitting flight socks and keep hydrated. See your GP if you have pain, swelling and a heavy ache in your leg.

Ear problems

Air pressure can cause ear pain, popping and muffled hearing. “The popping sensation is a result of a difference in pressure between the inner ear and the outside environment causing the ear drum to swell outward or be sucked inward (depending on whether pressure is rising or falling),” says Gordon Harrison, chief audiologist at ­Specsavers Audiology.

“This only becomes a problem when pressure changes quickly, so pressure inside and outside the ear don’t have time to equalise, for instance during takeoff and landing.”

To counteract and equalise the rapid change in pressure, you need to introduce as much air as possible to the ear.

■ What you can do: Try not to fly if you’ve got an ear or sinus infection. Don’t sleep during takeoff and landing. Swallow or yawn as often as possible, suck on a hard-boiled sweet or chew gum. Visit your doctor if hearing doesn’t return to normal several days after flying.

Infectious diseases

People might assume an aircraft’s a hotbed of infection, but up to half of the cabin air is recirculated through filters.

The Civil Aviation Authority describes these highly effective filters as “similar to those used in hospital operating theatres, to remove bacteria, viruses and other particles before it’s mixed with outside air from the air-conditioning units”.

The cabin air doesn’t lead to an increased risk of infection but like any public place where people are crammed together, germs and illness, such as flu or colds, can easily spread. For instance, as many as 40,000 droplets disperse when someone sneezes.

■ What you can do: Wash your hands or take a small hand sanitiser and avoid touching your eyes and mouth. If you’re the one feeling unwell, be considerate. Sneeze into the crook of your arm or cover your mouth when coughing and dispose of tissues before washing your hands.

Dehydration

The aircraft’s low humidity is the reason so many travellers can feel dry, itchy and in need of quenching their thirst on landing.

The Civil Aviation Authority reports the relative humidity in most air-conditioned buildings is between 40 and 70% compared to around just 20% in a plane. To put that in perspective the humidity in the Sahara Desert is reported to be around 25%.

“Dehydration can cause thirst, ­tiredness and dizziness, and urination is less frequent and darker,” says GP Dr Lisa Anderson (doctify.co.uk).

And choose the right plane. Next-generation Boeing Dreamliners and Airbus A350s have much higher levels of cabin humidity and better air quality.

■ What you can do: Drink plenty of water before, during and after the flight, keep an eye on what alcohol and caffeine you’re consuming, dampen contact lenses and remove them if you’re going to nap, and apply moisturiser.

Cosmic radiation

It might sound like science fiction but cosmic radiation is a reality. “We’re constantly bombarded by nuclei from our galaxy,” explains Christopher Mertens, Senior Research Physicist at NASA Langley Research Center, US.

“These high-energy particles are raining down into our atmosphere

all the time. It just so happens that commercial aircraft fly just under the peak of that radiation.” The most exposed flights are over the poles – those from the US to Europe or Asia.

“On a round trip using a high-latitude route you’ve received effectively two X-rays,” notes Mertens.

While there’s no imminent danger from cosmic radiation, according to the International Commission on Radiological Protection (ICRP), Mertens says: “Each exposure incrementally increases your probability of developing fatal cancer. Just like when you get an X-ray.

“With every one you receive, there’s an incremental probability that X-ray exposure will at some point down the road lead to cancer.”

■ What you can do: The ICRP recommends the level of planned exposure to radiation, like a flight, should be less than 1 mSv per year. That corresponds to five to 10 high-latitude flights a year. If you’re pregnant, try not to take more than two to three flights during your first trimester.

Aerial views of landmarks around the world (GES)

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